Commercially available freight tracking systems are based on a combination of a Global Positioning System (GPS) radio receiver and an in-vehicle back-link device for transmitting information to a central reporting station. The in-vehicle back-link device for transmitting information is most usually a mobile telephone and is sometimes a radio transmitter. The GPS receiver determines the precise location of the vehicle from transmissions received from overhead GPS satellites and the back-link device communicates this location back to a central reporting station. This GPS and back-link system provides precise information on the location of the vehicle, but unfortunately is relatively expensive to implement. It would be desirable, therefor, to have a freight tracking system that is less costly.
When a mobile telephone is outside its “home” subscription area it is said to be “roaming”. When roaming, a mobile telephone continually listens for a new “beacon signal” from a nearby base station or tower. Upon reception of a new beacon signal, the mobile telephone will identify itself and request registration on the “visited location” system to be able to receive and send calls. When the visited location system receives the identification information it sends an “authentication request” to the requesting mobile telephone's home system, which sends an appropriate “authentication response” to the visited location system. The visited location system then approves or disapproves user access and sends “notification of authentication or rejection” to the mobile telephone. An approved mobile telephone is then able to receive and send calls in the visited location; the home system will subsequently receive all necessary information for all calls such that it will be able to bill for “roaming”, long distance and other provided services. A rejected mobile telephone does not have access to service in the visited location. For more detail on roaming and beacon signals see “Beacon Signals: What, Why, How, and Where”, by S. Gerasenko, et.al., IEEE Computer, Volume 34, Number 10, October 2001, pp.108-110.